Faculty Publications
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Item A study on the effect of process parameters and scan strategies on microstructure and mechanical properties of laser directed energy deposited IN718(Elsevier Ltd, 2023) Thanumoorthy, R.S.; Sekar, P.; Bontha, S.; Balan, A.S.S.The present study focuses on understanding the effect of scan strategy on the microstructure and mechanical properties of LDED fabricated IN718 built at optimized process conditions from single track analysis. Initially, single track studies were conducted by varying laser power, scan speed, and feed rate (3 levels) to optimize process parameters for bulk deposition. Based on the dilution, aspect ratio, track continuity and melt pool shape, best process parameter were chosen for depositing bulk structures. Bulk rectangular specimens were fabricated using the LDED process for different infill rotation (0°, 45°, 67°, and 90°) at optimized process conditions. Infill rotation did not show any significant change in the density of the samples. However, grain size measurement from EBSD and SEM micrographs revealed a substantial difference in grain size between samples without infill rotation (0°) and samples with infill rotation (45°, 67°, and 90°). XRD and EDS mapping revealed higher the formation of secondary laves phases with infill rotation as a result of higher cooling rate. Similarly, melt pool shape and arrangement showed significant variation with different infill angles. Samples with 0° and 90° infill rotation exhibited strong crystallographic texture along the build direction. There was a significant variation in the microhardness and tensile strength of the build with variation in infill rotation. This variation in mechanical properties were attributed to grain size, LAGB's fraction, secondary phases, and crystallographic texture. © 2023 Elsevier B.V.Item Evaluation of functionally graded YSZ - IN625 clad without bond coat using laser directed energy deposition(Elsevier B.V., 2023) Likhwar, J.; Thanumoorthy, R.S.; Bontha, S.; Balan, A.S.S.M-CrAlY or M-Cr-based bond coats are used as a buffer layer to apply ceramic thermal barrier coatings (TBCs) to metal turbine blades. However, due to oxygen diffusion, thermally grown oxides grow over the bond coat material, leading to coating failure in the components. Therefore, this study attempts to fabricate a novel TBC-coated IN625 without bond coat material using a Functional grading approach. The findings from this study may pave the way for processing functionally graded ceramic materials using Laser Additive manufacturing techniques. This study also evaluates the performance of functional grading in joining dissimilar materials using small spot-size laser sources. In the present study, samples were fabricated for three sample conditions: S1: direct clad, S2: 25% linear grading, and S3: 50% linear grading. The interface between Yttria Stabilized Zirconia (YSZ) and IN625 for the S1 sample showed discontinuous and horizontal cracks along the interface due to steep variations in thermal properties. However, the interface of S2 and S3 samples showed good adhesion and a smooth transition in microstructure between IN625 and YSZ as a result of functional grading. SEM micrographs showed homogeneous YSZ distributions without segregation within the IN625 matrix. This was attributed to strong Marangoni flow as a result of the small spot-size laser beam used in this study. © 2023 Elsevier B.V.Item Hybrid additive manufacturing of ER70S6 steel and Inconel 625: A study on microstructure and mechanical properties(Elsevier Ltd, 2023) Rodrigues, J.P.; Thanumoorthy, R.S.; Manjhi, S.K.; Sekar, P.; Arumuga Perumal, D.A.; Bontha, S.; Balan, A.S.S.Hybrid Additive Manufacturing (HAM) is currently being explored because of its potential to achieve trade-off between build capacity and feature resolution. The present study aims at fabricating ER70S6-Inconel 625 (IN625) bimetallic clad using hybrid Wire Arc Additive Manufacturing (WAAM) and Laser Directed Energy Deposition (LDED) processes. Microstructure evaluation was performed at the cross section of bimetallic clad for distinct materials as well as the interface. WAAM built ER70S6 revealed equiaxed ferritic grains, whereas laser deposited IN625 region showed columnar dendrites with under developed secondary arms. However, the first layer of IN625 exhibited columnar dendrite with secondary arms due to the influence of diffused Fe from the base ER70S6 steel under the action of concentrated laser heat source, which was revealed by energy dispersive spectroscopy (EDS) maps. The measured microhardness across the cross section of the deposit showed values corresponding to inherent material system. The interface did not reveal presence of any intermetallic phases which was confirmed by hardness results and X-Ray diffraction. Shear test revealed superior bond strength between the two materials, maintaining average strength of 452 MPa. The fractography images exhibited fine dimples along with cleavages indicating mixed fracture characteristics. This additive manufacturing method explores a new dimension in multi-material fabrication which, when customized for different materials, serve critical areas in the aerospace and defence sector. © 2023 Elsevier LtdItem Sequential hybridisation of wire and powder-based additive manufacturing of Inconel 718: Mechanical and microstructural characterization(Elsevier Ltd, 2024) Mudakavi, D.; B Sreesha, R.; Thanumoorthy, R.S.; Anar, S.; Krishnan R, A.; Suryakumar, S.; Bontha, S.; M Adinarayanappa, S.The present study combines the Wire-based DED (W-DED) and Powder-based DED (P-DED) to achieve a high deposition rate and higher feature resolution, respectively, within the single component. The research puts forward a novel Wire-Power (WP) Hybrid DED process, which is realized by sequential deposition of feedstock in Wire and then in Powder form. Based on the deposition-extraction combination, three sample configurations, C1 (Y-X), C2 (X-X) and C3 (Y-Z), were fabricated and characterized for the mechanical properties and microstructural aspects. OM images revealed defect-free P-DED and W-DED interface, while the EBSD analysis showed grain size variations owing to differences in the cooling rates. The Ultimate Tensile Strength (UTS) values of C1 and C2 configurations are about 132.2 and 139.7 % higher in comparison to C3. Low cycle fatigue results showed that the C2 sustained a higher number of completely reversed cycles to failure in comparison to the other configurations. The impact energy absorbed by C3 is the highest, affirming the strong W–P interface. © 2024 Elsevier B.V.Item Process parameter optimization for laser directed energy deposition (LDED) of Ti6Al4V using single-track experiments with small laser spot size(Elsevier Ltd, 2024) Gonnabattula, A.; Thanumoorthy, R.S.; Bontha, S.; Balan, A.A.S.; Anil Kumar, V.A.; Kanjarla, A.K.Single-track experiments are routinely used in the optimization of process parameters in additive manufacturing processes. Most of the process parameter optimization studies use a laser spot size of 1 mm or more. Since laser spot size affects the input energy density and in turn the efficiency of the deposition process, it is important to develop process maps every time a laser of different spot sizes is used. In this work, we determine the process maps for a laser of 0.6 mm spot size. By combining the process maps and the metallographic inspection, we estimate the optimum process parameters (laser power, scan speed, powder feed rate) for building Ti6Al4V components using powder-based laser-directed energy deposition(LDED). Single-tracks corresponding to 64 different parameter combinations are deposited. After eliminating the process parameter combinations resulting in defective tracks, the optimum process parameters of 300 W laser power and 720 mm min−1 scan speed is established by considering the relationship between the process parameters and the geometrical features of the deposit. The experimental results are then used to calibrate the modeling parameters of a three-dimensional finite element model for simulating the deposition process. © 2024 Elsevier LtdItem Effect of Build Orientation on Anisotropy in Tensile Behavior of Laser Powder Bed Fusion Fabricated SS316L(Springer, 2024) Thanumoorthy, R.S.; Chaurasia, J.K.; Anil Kumar, V.A.; Pradeep, P.I.; Balan, A.A.S.; Rajasekaran, B.; Sahu, A.; Bontha, S.In the present study, Stainless steel 316L (SS316L) cylindrical specimens were fabricated at two different build orientations and two different laser powers using Laser powder bed fusion process (LPBF). Microstructural characterization such as XRD, SEM, EBSD analysis and tensile testing were carried out on fabricated specimens in stress relieved condition to understand the anisotropic behavior of LPBF printed specimens. Horizontally oriented specimens showed higher tensile strength when compared to vertically oriented specimens for both laser powers. XRD and EBSD phase maps did not reveal the presence of any secondary phases. However, build orientation and laser power affected the crystallite size of the samples. Bimodal grain structure comprising coarse columnar grains and fine equiaxed grains were observed from the micrographs. With variation in build orientation, there was a significant change in the average grain size of the specimens. High dislocation density was observed in horizontally oriented samples built at low laser power because of dislocation annihilation that can occur at high temperatures. However, EBSD analysis revealed random weak crystallographic texture which does not vary significantly with laser power or build orientation. Variation in grain size, grain morphology, sub-grain features and dislocation density are the reasons for the anisotropic tensile behavior observed in LPBF printed SS316L coupons in stress relieved condition. © ASM International 2023.Item Effect of surface remelting on the characteristics of IN718 components fabricated using laser powder directed energy deposition(Institute of Physics, 2024) Thanumoorthy, R.S.; Jadhav, S.V.; Oyyaravelu, R.; Bontha, S.; Balan, A.A.S.Laser Powder Directed Energy Deposition (LP-DED) fabricated components exhibit poor surface finish, necessitating additional post-processing steps prior to their practical application. Enhancing the surface quality of additively manufactured IN718 specimens through conventional post-processing methods is particularly challenging, given the material’s poor machinability and the complexity of the fabricated components. The current study is centered on comprehending the impact of Laser Surface Remelting (LSR) on the surface properties of Laser Powder Directed Energy Deposited (LP-DED) IN718 material. To gain insights into how remelting influences surface characteristics, remelting was carried out using various sets of parameters. The remelted zone exhibited a refined grain structure, leading to increased hardness. Moreover, significant reductions in surface roughness and residual stress were observed in the remelted samples. Regression analysis indicated that laser power played a pivotal role, with positive impact on surface finish and depth of influence but a negative impact on residual stress and hardness. Therefore, considering all the comparison metrics, remelting using laser power of 150 W and a scan speed of 1140 mm min−1 were found to yield optimal surface conditions. © 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.Item Thermal life assessment of laser powder-directed energy deposited NiCrAlY/CuCrZr bimetallic clad for rocket nozzle applications(Elsevier B.V., 2024) Thanumoorthy, R.S.; Urs, S.S.; Bontha, S.; Balan, A.S.S.To enhance the thermal life of rocket exhaust nozzles, the hot side of copper liners is coated with thermal barrier coatings (TBCs) to provide thermal insulation and oxidation resistance. However, interface failures often occur between M-CrAlY bond coats and nozzle liners due to significant differences in their thermal expansion coefficients (CTE). This study explores the use of Laser Powder-Directed Energy Deposition (LP-DED) to clad NiCrAlY onto a CuCrZr substrate, as the process offers localized heating which can offer better bond strength. Optimization trials were conducted using single and multi-track studies to identify optimal parameters. Due to the low energy absorption of the CuCrZr substrate to 1070 nm laser sources, cladding was performed at a high energy density of 135 J/mm2 with a 1.2 g/min feed rate to achieve defect-free clads with sufficient diffusion. The bulk of the NiCrAlY clads showed ??-Ni3Al, ?-NiAl, and ?-Ni phases, while Y4Al2O9 and Y2O3 oxides formed on the top surface due to aluminum and yttrium depletion at high temperatures. The clads exhibited cellular dendritic microstructures at the bulk region, and planar microstructures were observed at the dilution zone. EBSD-KAM maps showed higher dislocation density near the interface due to CTE mismatch across substrate and clad. Scratch tests confirmed strong adhesion with no interface cracks, though crack propagation was observed from the edges after 50 isothermal cycles, driven by copper erosion. With Cu diffusion, interface region exhibited a graded microstructure which could enhance CTE, improving compatibility compared to standard NiCrAlY alloys. © 2024 Elsevier B.V.Item Surface Characteristics of Low Plasticity Burnished Laser Directed Energy Deposition Alloy IN718(Springer, 2024) Mohanraj; Thanumoorthy, R.S.; Sekar, P.; Muthuchamy, A.; Bontha, S.; Balan, A.S.S.The research work focuses on a novel post-processing sequence to improve the surface integrity and residual stress characteristics of as-printed Inconel718 (IN718) samples. The as-printed IN718 samples are subjected to solution treatment at 1050 °C, two-step precipitation hardening (@ 720 °C for 8 h and @ 620 °C for 8 h), and low plasticity burnishing. Two different sequences were attempted. Sequence-1 involves solutionizing ? low plasticity burnishing followed by precipitation hardening, and sequence-2 includes solutionizing ? precipitation hardening followed by low plasticity burnishing. The experimental observations and detailed investigations revealed that the samples processed via sequence 2 exhibited a better surface finish. The microhardness of the samples of sequence 2 is 10% higher than their counterparts in sequence 1. The maximum residual stress of ?1375.33 MPa is obtained in sequence 1 as compared to the residual stress of ?1100.67 MPa in sequence 2. The influence of the processing sequences on the surface properties has been discussed in detail using the XRD and microstructural characterization supported with EBSD analysis. Graphic Abstract: (Figure presented.) © The Indian Institute of Metals - IIM 2024.Item Copper-graphene nanocomposite fabrication through LP-DED process: Powder preparation, characterization and printability studies(Elsevier Ltd, 2024) Sharma, S.; Thanumoorthy, R.S.; Bontha, S.; Balan, A.S.S.Copper and its alloys play a crucial role in various engineering applications due to their excellent conductive properties. However, their poor laser absorptivity and high conductivity make them a complex material to work with using laser additive manufacturing processes, hindering the ease of fabrication of precise and complex geometries. To overcome this challenge, graphene-reinforced copper powders were employed to enhance laser absorptivity. With graphene addition, there was a substantial increase in the laser absorptivity. The addition of graphene improved laser absorptivity from 15 % for pure copper to ~60 % in Gr-Cu composites. However, the flowability deteriorated at higher compositions, which could result from increased specific surface area due to graphene agglomeration and its nanoscale surface. The influence of graphene on the ease of fabrication employing laser powder-directed energy deposition was evaluated with a single-track and bulk deposition. A single-track study revealed that pure copper tracks were inconsistent and exhibited poor bonding due to their poor laser absorptivity. Meanwhile, graphene?copper composite tracks displayed stable melt pools and uniform tracks, which could result from enhanced absorptivity. Geometrically sound and defect-free Gr-Cu tracks were deposited using 750 W laser power with composite powders, while pure copper tracks at 950 W laser power deposition yielded defective tracks. However, a graphene percentage above 0.1 % resulted in the formation of keyhole porosity due to a significant enhancement in laser absorption (~60 %). A similar observation was made for bulk deposition, i.e., defect-free deposition for Gr-Cu composites ?0.1 % graphene and keyhole porosities in the deposition of 0.25Gr-Cu and 0.8Gr-Cu. © 2024 The Society of Manufacturing Engineers